The invention relates to a color display in the case of which it is possible by using at least two primary colors to represent color images with the aid of mixed colors of the primary colors.
Such color displays are required in very many technical applications, for example in monitors for computers or other technical equipment, in television screens for television sets or for telecommunication, in graphic machine displays etc.
In this case, the invention is directed at color displays in the case of which the brightness filter is backlit by a lamp. The brightness filter is spatially controllable, which means that how bright the filter is can be controlled in a spatially resolved fashion. In particular, this relates to the transmission through a brightness filter, but the reflection method can also be concerned. Usually, such brightness filters are liquid crystal devices, that is to say liquid crystal elements with a spatially resolved electric drive, or a system of liquid crystal elements.
In the case of conventional color liquid crystal displays, use is made of two-dimensional white light sources for backlighting and, as in the case of an electron beam color picture tube pixels in the required primary colors are used to produce images which, lacking spatial resolution of the individual pixels by the human eye, produce the impression of a color image with the aid of appropriate mixed colors of the primary colors. In this case, each pixel forms a color filter for the white light of the backlighting. The mixed colors are represented by spatially alternating, but simultaneous representation of the primary colors. It is felt as very disadvantageous that further losses occur over and above the low transmission of the liquid crystal devices owing to the color filtering. Furthermore, the geometrical dimensions of the individual primary color pixels are necessarily smaller than the spatial resolution of the color liquid crystal display, since in each case a set of primary color pixels of each primary color used together form a pixel.
As an alternative to this, color liquid crystal displays have also been developed which replace the spatial interlocking of the primary colors by a temporary sequential method in which the same pixel shines sequentially in the primary colors used with the respectively assigned brightness. The spatial averaging of the human eye in this case also produces the impression of a color image with the aid of appropriate mixed colors of the primary colors.
For this purpose, the backlighting itself must form the temporal sequence of the primary colors. Good results have been obtained with color liquid crystal displays which are backlit by colored light-emitting diode panels. Given the same structural size, it is possible to achieve a better spatial resolution than in the case of the systems previously described; furthermore, the light loss is eliminated by the color filter action of the individual primary color pixels.
The invention represented here is based on the technical problem of specifying a color liquid crystal display which is improved by comparison with the prior art.
For this purpose, the invention consists in a color display having a spatially controllable brightness filter and a lamp for backlighting the brightness filter, the display being designed for the purpose of producing images in at least two primary colors alternating sequentially in time, in order overall to create the impression of a color image with mixed colors of the primary colors, characterized in that the lamp is a discharge lamp with at least one dielectrically impeded electrode. Thus, according to the invention the basic principle of the temporary sequential superimposition of primary color images is adopted, although instead of backlighting by light-emitting diodes use is made of a discharge lamp which is designed for dielectrically impeded discharges. This discharge lamp has the substantial advantage that although it has a switching endurance comparable to that of light emitting diodes, that is to say can be used given the estimated number of switchings, approximately 1010 cycles and beyond, to be expected in the service life of a lamp, by contrast with light emitting diode arrangements it can be used without substantial technical problems and in conjunction with very favourable costs even in large formats. Furthermore, very bright background lighting systems can be produced with the aid of dielectrically impeded discharges. Finally, the dielectrically impeded discharges are short lived and can therefore also be switched quickly, with the result that, in particular, it is possible to produce very short switching times in conjunction with recent display systems based on ferroelectric or antiferroelectric liquid crystals.
It is to be noted that in this application the term of spatially controllable brightness filter is used and meant in the most general sense. The liquid crystal devices discussed in more detail below constitute only the application which is currently technically customary in this regard. However, the invention is likewise directed to other embodiments, including those still to be developed in future. All that is essential is that the brightness filter modulates the brightness of the image to be displayed, thus being responsible, as it were, for the spatial part of the image representation (and, moreover, possibly also for the color structure of the image by the weighting of the primary colors at the relevant point).
The lamp is preferably based on the Xe excimer system, and thus generates a VUV excitation for appropriate fluorescent materials. Such discharges can be operated particularly well in a pulsed fashion, as is shown in a prior application whose disclosed content is included here (European Application 94 911 103.3). Such a pulsed mode of operation can be technically realised with the aid of electronic ballasts such as are described in the two prior applications 198 39 329.6 and 198 39 336.9. The disclosed content of these two applications is also included here. Precisely in the case of the flat applications, under consideration here in monitors or television screens, the pulsed mode of operation can be electronic ballasts described in the quoted applications offer very bright and at the same time very energy-efficient backlighting systems.
So far, primary colors have been talked of unspecifically in the plural. In fact, the invention already functions with the aid of two primary colors which can be used to give the impression of mixed colors. However, color displays in the narrow sense require at least three primary colors which are also usually employed, for example blue, red and green.
Furthermore, the invention in no way excludes the use of more than three primary colors as well, for example in order to optimize the color rendition.
As already previously stated, it is possible with the aid of discharge lamps for dielectrically impeded discharges in conjunction with modern liquid crystal systems to realise very short switching times, and this has the advantage of a flicker free and quasi continuous representation of fast moving images as well. According to the current prior art, cycle times, that is to say time periods per primary color, of the order of magnitude of approximately 1 ms and below are realistic. The invention is preferably directed to cycle times of at most 10, better 7, even better 4, and particularly 2 ms, or less.
Precisely with regard to short cycle times, it is advantageous to use a TFT matrix (thin film transistor matrix) for driving the liquid crystal device. However, the invention is not limited to this technology, firstly because short switching times need not be in the forefront with every application, and secondly because improvements are not to be ruled out even in the case of alternative technologies, and thirdly with regard to future technologies, currently not yet known, for driving liquid crystal displays.
Many variants are conceivable in principle as regards the concrete geometric shape of the discharge lamp. The invention is directed, in particular, to three possibilities, which are also represented further below as exemplary embodiments. In the case of the first possibility, the discharge lamp actually consists of a plurality of individual discharge lamps which respectively have an essentially elongated shape, in particular a tubular shape. Each of the elongated individual discharge lamps generates a dedicated dielectrically impeded discharge and is assigned to a specific primary color, that is to say to an appropriate fluorescent material. For this purpose, the fluorescent material can be fitted on the lamp itself or in the immediate vicinity on another element. Overall, the individual discharge lamps form an alternating arrangement in which all primary colors used occur in a spatially interlocked fashion.
This interlocking and the number of the individual discharge lamps for each primary color should be designed so as to produce as homogeneous as possible a luminance for each primary color, possibly with the assistance of optical aids such as diffusers. It is possible in this way to avoid strip-shaped or otherwise shaped color casts of the displayed image. For example, the alternating arrangement of the tubular discharge lamps can also be provided in a light box which is coated in a diffusely reflecting fashion. However, it is also conceivable to compensate irregularities by filters or else by driving the brightness filter.
The second possibility consists in the use of only a few individual discharge lamps which are, however, likewise essentially elongated, that is to say tubular, in particular. The light from these individual discharge lamps is launched into an optical conductor device which ensures homogeneous distribution over the surface of the liquid crystal device. For this purpose, the surface of the liquid crystal device corresponds essentially to that of the optical conductor device. For example, appropriate tubular discharge lamps can be provided at one, two or more lateral edges of a rectangular optical conductor device. Each individual one of the discharge lamps has along its length an alternating sequence of fluorescent materials corresponding to the primary colors used, and corresponding sections which can be driven electrically in groups.
In the case of the third variant, a flat lamp for dielectrically impeded discharges has a system of spatially interlocked electrode groups which can be operated separately and are respectively assigned to fluorescent sections for the primary colors. The separate operability does not presuppose that all the electrodes must be divided into groups. For example, it can suffice to split up only the cathodes or the anodes, or in the case of bipolar operation only the electrodes of a sign group, into electrically separate groups. A similar result is obtained as for the first variant, but this is done with a single discharge lamp.
As already indicated, it is preferred in each of the three cases to operate with additional optical devices for the purpose of homogenization, for example with diffusely scattering elements between the lamp and liquid crystal device or inside the lamp, or with diffusely reflecting surfaces.
Inhomogeneities in the light emission of the lamp can, however, be compensated by the liquid crystal device, specifically both permanently, for example by an appropriate pixel structure or by influencing the transmission in some other way, and electrically, that is to say by superimposing a compensating filter pattern.
The invention is represented in detail below with the aid of three exemplary embodiments. Individual features disclosed in this case can also be essential to the invention in other combinations. In detail: